As a cathode active material contained in a positive electrode of a lithium ion secondary battery, a lithium-containing composite oxide, particularly LiCoO2, is well known. However, in recent years, for a lithium ion secondary battery for portable electronic instruments or for vehicles, downsizing and weight saving are required, and a further improvement in the discharge capacity of a lithium ion secondary battery per unit mass of the cathode active material (hereinafter sometimes referred to simply as the discharge capacity) is required.
As a cathode active material which may be able to further increase the discharge capacity of a lithium ion secondary battery, a cathode active material having high Li and Mn contents i.e. a so-called lithium rich cathode active material has attracted attention. As a lithium rich cathode active material capable of obtaining a lithium secondary battery having a large discharge capacity, the followings have been proposed.
(1) A cathode active material particulate powder which is made of a compound having a crystal structure with space group R-3m and a crystal structure with space group C2/m, wherein the compound is a composite oxide comprising at least Li, Mn and boron and Co and/or Ni, the relative intensity ratio (a)/(b) of (a) the intensity of the maximum diffraction peak at 2θ=20.8±1° in the figure of the X-ray powder diffraction using a Cu—Kα ray of the cathode active material particulate powder to (b) the intensity of the maximum diffraction peak at 2θ=18.6±1°, is from 0.02 to 0.5, the content of Mn of the cathode active material particulate powder is at least 0.55 in Mn/(Ni+Co+Mn) by the molar ratio, and from 0.001 to 3 wt % of boron is contained (Patent Document 1).
(2) A cathode active material particulate powder made of a compound having at least a crystal structure with space group R-3m and a crystal structure with space group C2/m, wherein the compound is a composite oxide comprising at least Li, Mn, an element A (at least one element selected from the group consisting of Si, Zr and Y) and Co and/or Ni, the relative intensity ratio (a)/(b) of (a) the intensity of the maximum diffraction peak at 2θ=20.8±1° in the figure of the X-ray powder diffraction using a Cu—Kα ray of the cathode active material particulate powder to (b) the intensity of the maximum diffraction peak at 2θ=18.6±1°, is from 0.02 to 0.2, the content of Mn of the cathode active material particulate powder is at least 0.55 (Mn/(Ni+Co+Mn)) by the molar ratio, from 0.03 to 5 wt % of the element A is contained, the tap density is from 0.8 to 2.4 g/cc, and the compressed density is from 2.0 to 3.1 g/cc (Patent Document 2).